1. Field of the Invention
The present invention relates to a silicon-controller rectifier, particularly to a symmetric bidirectional silicon-controlled rectifier, which has a small area and a high electrostatic-discharge protection capability. The present invention applies to an ESD protection element for I/O signals with both positive and negative voltage level.
2. Description of the Related Art
With the advance of semiconductor technology, the dimensions of MOS (Metal Oxide Semiconductor) devices have been reduced to a submicron or even deep-submicron scale. The submicron or deep-submicron technology uses so thin a gate oxide layer that only a few volts higher voltage is enough to cause gate oxide damage. In general environments, electrostatic voltage maybe reach thousands or even several ten thousands volts, which will damage integrated circuits (IC). Therefore, once having accumulated to a given amount in IC, electrostatic charge should be released by ESD device. The silicon-controlled rectifier, which has a low turn-on resistance, low capacitance, low power consumption and high-power current conduction capability, is exactly an effective ESD (Electro-Static Discharge) protection element for IC.
Currently, the bidirectional silicon-controlled rectifier (SCR) has become the mainstream in the market of the ESD protection circuits for I/O signals with both positive and negative voltage level, and many researches are also dedicated to the bidirectional silicon-controlled rectifier. U.S. Pat. Nos. 6,258,634, 6,365,924 and 7,034,363 all disclosed symmetric bidirectional silicon-controlled rectifiers. As the silicon-controlled rectifiers disclosed in the abovementioned patents are all directly fabricated on a silicon substrate, they have lower breakdown voltages and can only apply to generic IC processes. A U.S. Pat. No. 6,960,792 disclosed an symmetric bidirectional silicon-controlled rectifier with annular layout, which consumes a larger layout area. Furthermore, the trigger speed is also deeply influenced by the structure thereof. Therefore, the device proposed in U.S. Pat. No. 6,960,7922 cannot provide an effective ESD protection function. A U.S. Pat. No. 5,072,273 disclosed a low trigger voltage silicon-controlled rectifier. However, it can only operate unidirectionally. The proposed structure cannot apply to an ESD protection circuit for I/O signal with both positive and negative voltage level.
Accordingly, the present invention proposes a novel symmetric bidirectional silicon-controlled rectifier to overcome the abovementioned problems and drawbacks.